Chapter 9, Problem 9.31P

Interpretation Introduction

Interpretation:

From the given data $\Delta H_{\text{lattice}}^{\text{°}}$ of ${\text{MgF}}_2$ is to be calculated. Also, the magnitude of the lattice energy of ${\text{MgF}}_2$, $\text{NaCl}$ and $\text{LiF}$ is to be compared.

Concept introduction:

Lattice energy is the amount of energy released when the ions combine to form an ionic compound. It is determined to measure the strength of the ionic interactions present in the lattice. A high negative value of lattice energy is the reason behind the formation of ionic compounds.

The lattice energy of compounds depends upon generally three factors as follows:

1. Ionic Size

2. Ionic Charge

3. Arrangement of ions in the lattice

Lattice energy depends upon the ionic interaction between the ions present in the lattice. Therefore, lattice energy decreases with the increase in ionic size and decrease in the ionic charge.

Lattice energy can be calculated through Hess’s law. Hess’s law is used to calculate the enthalpy change of an overall reaction that can be derived as a sum of two or more reaction. According to Hess’s law $\Delta H$ of an overall reaction is equal to the sum of the enthalpy change for each individual reaction.

$\Delta H_{\text{overall rxn}}=\Delta H_1+\Delta H_2+\mathrm{.......}+\Delta H_{\text{n}}$